Process for preparing dialkylzinc compounds from alkylbromide

ABSTRACT

A DIRECT PROCESS FOR PREPARING DIALKYLZINC COMPOUNDS, WHEREIN ZINC ALLOYED WITH SODIUM, POTASSIUM OR LITHIUM IS REACTED WITH ALKYLBRONIDE IN THE ABSENCE OF MOISTURE TO PRODUCE A DILKYLZIN COMPOUND.

United States Patent 3,641,081 PROCESS FOR PREPARING DIALKYLZINCCOMPOUNDS FROM ALKYLBROMIDE Schrade F. Radtke, New Canaan, C0nn.,assignor to International Lead Zinc Research Organization Inc., NewYork, N.Y. No Drawing. Filed Jan. 21, 1969, Ser. No. 792,852 Int. Cl.C07f 3/06 US. Cl. 260-4293 12 Claims ABSTRACT OF THE DISCLOSURE A directprocess for preparing dialkylzinc compounds, wherein zinc alloyed withsodium, potassium or lithium is reacted with alkylbromide in the absenceof moisture to produce a dialkylzinc compound.

This invention relates to a direct process for preparing dialkylzinccompounds.

Diorganic compounds are generally prepared by two methods. The firstmethod reacts a zinc halide with a reactive organometallic, such aslithium or magnesium, in an ether solvent. The second method involves adirect synthesis starting from metallic zinc or zinc alloys with alkyliodides. Mixtures of alkyl bromides and iodides have also previouslybeen used in combination with zinccopper alloy.

The first method is disadvantageous in that it requires two separatesteps to be performed. In the first step the lithium or magnesiumreagent must be formed, and in the second step the actual formation ofthe organozinc compound takes place. An additional disadvantage is thepresence in the reaction of flammable solvents. The separation of thesolvents from the product is not always ealiilly achieved, especiallywith the utilization of lower a ys.

The second method is more attractive in that only one step is requiredand the use of solvents is not necessary, although recent attempts tofind improved conditions have utilized solvents with'high dielectricconstants, such as dimethylformamide. The use of solvents, however,makes it impossible to separate solvent from the product.

It has been suggested that the reaction of ethyliodide and the fusionproduct of zinc with a large proportion of sodium will result in adiethylzinc compound. The i0- dide material, however, is expensive, andthe presence of free sodium in the zinc alloy is hazardous and diflicultto work with.

Since metallic zinc alone does not react with alkyliodide or bromide toproduce more than minimal amounts of the dialkylzinc even under idealconditions, attempts have been made to use a zinc-copper alloy or fusionproduct. Unless extreme precautions are taken, however, the reaction maynot start for several hours. If after the start of the reaction thereactionv mixture is cooled too low, the reaction may stop entirely andis difficult to begin again. More importantly, it is recognized that theyields are then much lower. An additional disadvantage is the necessityof using expensive copper and alkyliodides as startingmate'rials.

It is therefore an object of this invention to provide a method forpreparing dialkylzinc compounds by direct synthesis, andmoreparticularly to provide such a process utilizing less expensivestarting materials.

According to the invention it has been found that zinc alloyed withsodium, potassium, or lithium will react with alkylbromide to synthesizedialkylzinc compounds. The reaction is believed to follow the generalformula:

3,641,081 Patented Feb. 8, 1972 The reactions are performed by refluxinga mixture of the finely divided zinc alloy and the alkylbromide. Thereaction should be carried out in the absence of moisture and thereforean inert atmosphere is preferred, such as nitrogen, argon, or carbondioxide. The reactions may be carried out at atmospheric pressure,unless the starting materials are too volatile, in which case pressureshould be applied as with MeBr and EtBr.

It is preferred to use an excess of the zinc alloy since this increasesthe speed and the yield of the reaction. A molar ratio of zinc toalkylbromide of between 2:1 and 1:1 has been found satisfactory. It isunnecessary to perform the reaction in the presence of a solvent, sinceit is usually very difficult, if not impossible, to separate the productfrom the solvent.

The reaction usually begins a few minutes after the starting materialshave made contact with each other, especially if substantially alltraces of moisture have been removed from the atmosphere and theapparatus. Heating of the reaction mixture may be necessary to begin thereaction, but if the reaction is exothermal refluxing temperature ismaintained without the application of external heat. In the event thatthe reaction is not exothermal, external heating may be applied tomaintain the reflux temperature. It has been found that a temperature inthe range of 40-l80 C. is satisfactory. A temperature range of l00-l40C. is preferred.

The reaction is considered complete when refluxing stops. The flask isconnected with a distilling head and the contents of the flask aredistilled under reduced pressure. Distillation of the product mixturevaporizes the dialkylzinc compound which is captured as a distillationproduct.

The zinc alloys which have proved themselves reliable for purposes ofthe invention are the lithium, potassium and sodium alloys. The alloysmay be formed by fusing the metals together in a steel crucible under aninert atmosphere, for example, argon. The cooled melt is machined tofine particle size. Usually turnings may be used, but if the alloy isvery brittle a sandlike material is obtained upon machining. The shotmethod may also be utilized to prepare the alloys on an industrialscale.

The maximum amount of sodium which can be alloyed with zinc is one atomof sodium per twelve atoms of zinc. This corresponds with an alloycontaining three wt. percent sodium, the balance being zinc.

Table I contains data showing the yield of dialkylzinc in relation tothe percentage of sodium in the alloy.

TABLE I Yield Wt. percent Alkyl- RzZn, Na in alloy bromide percentDetails 3 n-PrBr 67 min. reflux. 3 l-PrBr 39 Reaction complete after 10min. reflux. 3 n-BuBr 72 Exotherrnal reaction complete mm. 1 n-BuBr 5860 min. reflux. No gas formation. 3 n-C5Hu-Br 80 Exothermal reaction 10min.

complete. 3 iso-O5H Br 46 Kept at for 15 min. 8. Br 72 2. 67 1. 58 0. 50

It is apparent that the yield of dialkylzinc decreases as the sodiumcontent of the alloy decreases. The 2% sodium/ zinc alloy gives,however, no significantly lower yields than the 3% sodium/zinc alloy.The zinc alloy containing as little as 1% sodium also gives good resultswith the alkylbromide. =Upon further lowering of the sodium content inthe zinc alloy, the yields of dialkylzinc gradually decrease to thoseobtained using pure zinc. In view of the higher yields obtainable withthe 2% and 3% sodium zinc alloy it is preferred that the percentage ofsodium lie between 2 and 3 wt. percent.

Zinc alloyed with potassium is obtained by fusing zinc and potassiumunder an inert atmosphere, such as nitrogen, and machining in the usualway. The alloy is very reactive and should be kept in an inertatmosphere.

The maximum amount of potassium in the zinc/potas sium alloy isapproximately 5 wt. percent, which corresponds to a molar ratio of 12moles zinc to 1 mole potassium. .The reaction with the alkylbromide iscarried out in the same manner as with the zinc/sodium alloy. Oncebegun, the reaction continues spontaneously upon addition of thebromide.

-A comparison of the yield in dialkylzinc as a function of the percentof potassium in the zinc alloy is shown in Table H.

TABLE 11 Wt. percent Alkyl- Yield K in alloy bromide percent; Details 5BuBr 58 Starts spontaneously. Finally kept at 140 for 60 min.

It is evident that the potassium content of the alloy may be reducedwithout loss of activity. The zinc/potassium alloys are easily handledand machined to small particle size. As appears from the results inTable II, the 2% potassium/zinc alloy is well suited for the directsynthesis of zinc dialkyls, although the yields in general are somewhatlower than for the zinc/ sodium alloys. The recommended range would be0.5-5.0 wt. percent potassium with 1.5 to 2.0 wt. percent potassiumpreferred.

Reactions with the zinc/potassium alloys started without any appreciableinduction period and were easily controlled.

In formulating the zinc/lithium alloys it was found important to controlclosely the homogeneity of the alloy. A well-defined alloy containing2.0 wt. percent lithium was reacted with butylbromide to produce a yieldof 52% dibutylzinc compound. Experiments conducted with a 20%lithium/zinc alloy produced side reactions and the yield of dialkylzincisolated was invariably lower. For example, with a 20% lithium/zincalloy the reaction with butylbromide yielded 23% dibutylzinc.

It was found that a useful lithium range is 1 to wt. percent lithium inthe zinc alloy, with a preferred content of approximately 2 wt. percentlithium.

Ternary alloys may be used to provide good yields of dialkylzinccompounds. The amalgamation of the zinc/ sodium alloy by treating the 3%sodium/zinc alloy with HgCl in tetrahydrofuran provides approximatelythe same yield as using the zinc/sodium alloy without the Hgcl An alloycomposed of 2.6% sodium, 1.1% mercury and the rest zinc was reacted withbutyl'brornide and yielded 52% dibutylzinc.

A11 alloy composed of 2.3% sodium, 0.8% lithium, and the rest zinc wasreacted with butylbromide and yielded 60% dibutylzinc.

The synthesis according to the invention may be carried out with bothnormal and branched chain alkylbromides. Since longer chaindialkylzincs, for example, where R is greater than C have limitedthermal stability, a direct synthesis involving thermal cracking ofprimarily formed RZnBr to give R Zn+ZnBr is not practical. At the otherend of the scale, the boiling point of methyl bromide is low andrequires reaction under higher pressure conditions than atmosphericpressure.

The alkyl group may be unsaturated, #but the double bond should be morethan two carbon atoms removed from the zinc atom. Both Zn(CH CH =C I-Iand Zn(CH 'CI-I=CHCH are thermally unstable. Cornpounds with a CEC groupcan be prepared if the CEC group is internal, Compounds with a terminalCECH group decompose due to the acidity of the.,ter. rninal hydrogen.

The following examples illustrate the process of the invention and arenot intended to limit in any way the scope of the invention.

EXAMPLE 1 n-Bu Zn from Zn/Na alloy (3% Na) and n-BuBr (n-C H Zn from Zn/Na alloy (3% Na) and n-C H Br In 500 ml. flask: 20.4 g. 'Zn-3.0'Na (0:30g. at.), 22.7 g. n-pentylbromide (0.16 mole) and a crystal of 1 Themixture was gradually warmed to when an exothermal reaction occurredwhich was completed in about 10 minutes. The product isolated bydistillation under reduced pressure (B.P. 99100/1l'mm.) consisted ofnearly pure di-n-pentylzinc (gas chromatographic analysis of theproducts obtained after hydrolysis of a small sam-' ple in ether showedthe presence of n-pent'ane and only traces of n-pentylbromide). 1 1

EXAMPLE 3 (Iso-C H Zn from Zn/Na alloy (3% Na) and iso-C H Br I In 500ml. flask: 15.5 g. Zn-3.0 Na (0.23 g. at), 17.2 g. (0.11 mole) ofiso-amylbromide and a crystal of I The temperature of the mixture wasraised by gradually heating the oil bath to An exothermal reactionstarted which was completely in circa 15 min. The-product (5.95 g.)isolated upon heating (oil bath temp. 120") the reaction product invacuo (2 mm. Hg) contained some 2,7-dimethyloctane (the Wurtz couplingproduct of iso-amylbromide) as, appeared from a gas chromatoe graphicanalysis of the hydrolysis products). The yield of di-isoamylzinc afterrefractionation, B.P. 64-,689/ 2 mm.) was 5.48 g. (46% of theory).

EXAMPLE 4 n-Bu Zn from Zn/K alloy (5% K) and n-BuBr Upon dropwiseaddition of n-BuBr. to 40.6 g. 'Zn-5.0 K (0.30 g. at. Zn) an exothermalreaction started. The dropwise addition was continued until 32 ml. BuBr(0.30 mole) had been added. At the same time the temperature of the oilbath was gradually raised to 150 and kept-at that temperature for '60min. Volatile products were then removed in vacuo. In this way 13.2 g.=of n-Bu Zn were obtained.

EXAMPLE 5 I 1 n-Bu Zn from ZnK alloy (2% K) and n-BuBr A mixture of 29.0g. of finely divided Zn-2.0 K. alloy (0.44 g. at. Zn) and 5.0 ml. ofn-BuBr was gradually warmed up (to oil bath temp, of When reaction hadstarted -18 ml. of n-BuBr (0.22 mole n"-BuBr.in total)v was addeddropwise' After a further 30 min hea'ting period 10.8 g. n-Bu Zn (55% oftheory) was isolated by distillation in vacuo.

EXAMPLE 6 n-Bu Zn from Zn/Li alloy (2.0% Li) and n BuBr The reaction of45.4 g. Zn-"2.0 Li (0.68 g: ,at. Zn and 36 ml. of n-BuBr (0.34 mole)carried out as in Example 5 aiforded 15.8 g. of n-Bu Zn (52% of theory).

EXAMPLE 7 Di-n-buten-3-ylzinc from Zn/Na alloy (2% Na) and 4-bromobutene-1 A mixture of 29.8 g. Zn-2.0 Na (0.44 g. at. Zn), 29.7 g.(0.22 mole) of 4-bromobutene-1 and a few crystals of I were graduallywarmed up till an exothermal reaction occurred. When the reaction hadsubsided heating (oil bath-140) was continued for min. Upon distillationin vacuo 10.5 g. of (CH=CHCH CH Zn were isolated. Yield 54%.

I claim:

1. A process for preparing dialkylzinc compounds comprising:

(a) reacting in a moisture-free atmosphere:

*(i) an alloy of zinc having at least one metal selected from the groupconsisting of sodium, potassium, and lithium, the amount of metal in thealloy being about 1 to about 3 weight percent sodium, about 0.5 to about5.0 weight percent potassium and about 1 to about 10 weight percentlithium, with the remaining percentage being zinc, with (ii) a reagentconsisting essentially of alkylbromide in a molar ratio of at least 1 to1, zinc to alkylbromide, wherein the alkyl radical is selected from thegroup consisting of saturated and unsaturated alkyl radicals containingfrom 1 to -8 carbon atoms; and

('b) refluxing the mixture of said alloy and alkylbromide reagent at atemperature of between 40 C. and about 180 C. until the refluxingceases.

2. A process as described in claim 1, wherein the refiuxed mixture isdistilled under reduced pressure in the absence of moisture to separatethe volatile dialkylzinc compound from the in-volatile residue.

3. A process as described in claim 1, wherein the zinc alloy is inparticulate form.

4. A process as described in claim 1, wherein the zinc alloy containsapproximately 2-3 wt. percent sodium.

5. A process as described in claim 1, wherein the zinc alloy containsapproximately l-2 wt. percent potassium.

6. A process as described in claim 1, wherein the zinc alloy containsapproximately 2 wt. percent lithium.

7. A process as described in claim 1, wherein the zinc alloy containsboth sodium and lithium in addition to Zll'lC.

8. A process as described in claim 1, wherein the alkylbromide isselected from the group consisting of methylbromide and ethylbromide,and the alkylbromide is re fiuxed under a pressure above that ofatmospheric pressure.

9. A process as described in claim 1, wherein the refluxing temperatureis approximately C. to C.

10. A process as described in claim 1, wherein the molar ratio of thezinc alloy to the alkylbromide in the mixture is from 2:1 to 1:1.

11. A process as described in claim 1, wherein dialkylzinc is added tothe reaction mixture to remove traces of moisture.

, 12. A process as described in claim 1, wherein the zinc alloy containsapproximately 1.1 wt. percent mercury.

References Cited Reith et al., Annalen, vol. 123, pp. 245-48 (1862).Alexeyeff et al., Compt. rendu., vol. 58, pp. 171-73 (1864).

Nesmeyanov et al., Methods of Elemento-Organic Chemistry, North-HollandPubl. 00., Amsterdam, vol. 3, pp. 8 to 14, 24 and 25 (1967).

TOBIAS E. LEVOW, Primary Examiner S. SN-EED, Assistant Examiner UNITEDSTATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent: No. 3,641,081Dated February 8, l972 Inventofls) Schrade F. Radtke and that saidLetters Patent are hereby corrected as shown below:

It is certified that error appears in the above-identified patent Column1, line 48, before "material" insert -star ting-;

Zn" should be Column 5, line 10, "(CH=C HCH2CH2) Signed and sealed this1st day of August 1972.

(SEAL) Attest:

ROBERT GOTI'SCHALK Commissioner of Patents EDWARD M.FLETCHBR,JR.Attesting Officer

